Car exhaust primarily contains harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC). Environmental concerns and government regulations have led efforts to remove these noxious combustion products from vehicle exhaust by conversion to more benign gases such as carbon dioxide (CO2), nitrogen (N2), and water (H2O). In order to accomplish this conversion, the exhaust gases must pass through a treatment system that contains materials that can oxidize CO to CO2, reduce NOx to N2 and H2O, and oxidize hydrocarbons to CO2 and H2O.
Emission regulations and standards are becoming more and more stringent worldwide, especially for NOx emissions. Two competing exhaust technologies to reduce the amount of NOx released into the atmosphere are Lean NOx Traps (LNT) and Selective Catalytic Reduction (SCR). LNTs absorb, store, or trap nitrogen oxides during lean-burn engine operation (i.e., when excess oxygen is present), and release and convert these gases when the oxygen content in the exhaust gas is reduced. An example of an LNT system can be found in International Patent Application PCT/US2014/061812 (WO 2015/061482)and U.S. Provisional Application 61/894,346 (now expired), which are hereby incorporated by reference in their entirety. On the other hand, SCR units reduce nitrogen oxides regardless of the amount of oxygen in the exhaust gas. However, SCR units cannot properly reduce NOx emissions at low operating temperatures, for example, temperatures below 200° C.
Unfortunately, a significant portion of pollutant gases emitted by internal combustion engines are produced when the engine is initially started (“cold-start”), but before the catalytic converters, LNTs, or SCR units in the emissions system have warmed up to their operating temperatures. In order to reduce harmful emissions during the cold-start phase, such as that of a light-duty diesel or gasoline vehicle (for example, an automobile or light truck), washcoats that contain temporary storage for pollutants can be used to coat the substrate used in the catalytic converter of the vehicle. After the catalytic converter heats up to its operating temperature, known as the light-off temperature (the temperature at which the conversion rate reaches 50% of the maximum rate), the stored gases are released and subsequently decomposed by the catalytic converter.
A high light-off temperature is undesirable, as many vehicular trips are of short duration, and during the time required for the catalytic converter to reach its operating temperature, pollutants must either be released untreated to the environment, or stored in the exhaust system until the light-off temperature is reached. Even if pollutants are trapped effectively prior to light-off, the catalytic converter may not reach operating temperature if multiple successive short trips are made. Thus, the washcoats used for storage may become saturated, resulting once again in the release of pollutants to the environment.
In addition, the exhaust temperature of an engine or vehicle can vary depending on the type of engine or vehicle. Thus, the operating temperature of the catalytically active material or the operating temperature of the SCR unit can vary depending on the engine and vehicle. For example, large engines (e.g., greater than 2.5 Liters) typically run colder than small engines (e.g., less than 2 Liters). Accordingly a tunable material used for storage of pollutants, where the release temperature can be adjusted or tuned up or down to accommodate varying operating temperatures in engines or vehicles, is desirable.
Commercially available catalytic converters use platinum group metal (PGM) catalysts deposited on substrates by wet chemistry methods, such as precipitation of platinum ions, palladium ions, or platinum and palladium ions from solution onto a substrate. These PGM catalysts are a considerable portion of the cost of catalytic converters. Thus, any reduction in the amount of PGM catalysts used to produce a catalytic converter is desirable. Commercially available catalytic converters also display a phenomenon known as “aging,” in which they become less effective over time; the light-off temperature starts to rise as the catalytic converter ages, and emission levels also start to rise. Accordingly, reduction of the aging effect is also desirable, in order to prolong the efficacy of the catalytic converter for controlling emissions.